1. Field of the Invention
The present invention relates generally to wireless communication devices, and more specifically to a relatively compact antenna (PIFA) suitable for use in such devices.
2. Description of Related Art
Wireless communication equipment, such as cellular and other wireless telephones, wireless network (WiLAN) components, GPS receivers, mobile radios, pagers, and other wireless devices are enjoying increasing popularity in the contemporary marketplace. One reason for their increasing popularity is the large number of applications that such devices are now capable of supporting. Additional reasons include enhanced user interfaces, longer battery life, increasing affordability, and improved operability, among others.
One critical feature of wireless devices not often contemplated by their users is the antenna, which provides a region of transition between a signal in a guided wave within the device and a free space wave. After all, it is the antenna, which can be used to both transmit and receive information signals, that allows the wireless device the ability to communicate across a wide range. Antenna technology continues to advance rapidly and such advances are instrumental in enabling higher performance and smaller packaging in wireless devices. For example, enhancements in antenna technology can yield increased performance in terms of higher signal strength, improved reception of weaker signals, longer battery life, increased (or narrowed, if desired) bandwidth and smaller packaging.
Perhaps the most common antenna is a simple whip antenna, having a length that is typically λ/8, λ/4 or λ/2 (where λ is the wavelength). The popularity of whip antennas is attributed to their low cost, ease of manufacture and simplicity of implementation. They operate over a wide bandwidth and provide a radiation pattern that is well suited to mobile applications. In place of whip antennas, helical antennas are sometimes used in wireless devices. A helical antenna includes one or more conductive radiators wound in the shape of a helix. An feature of the helical design is its small size, and, for certain applications such as GPS receivers, its circular polarization. Although they enjoy widespread use, whip and helical antennas protrude from the package and are prone to breakage if the phone is mishandled. Also, their length tends to interfere with the form factor of the device, especially for handheld or portable applications.
To avoid some of the drawbacks associated with whip and helical antennas, conventional systems often utilize what are commonly known as microstrip, or patch, antennas to obtain modest performance from a relatively small package. Such antennas utilize a conductive material formed in a stripline, rectangular, circular or other shape, and disposed on a dielectric substrate of certain dielectric value and thickness. The shape of the conductor is chosen to achieve the desired resonant frequency and radiation pattern. Selecting a lower substrate permittivity and a larger patch size yields a higher antenna efficiency. Impedance matching is optimized by selecting an appropriate location on the patch for the feed point. Excitation via the feed results in a charge distribution on the underside of the patch and the ground plane. The patch antennas allow a great flexibility in antenna and wireless-device design, as they are cost-effective, easily manufactured, and can be conformed to the shape of the wireless device.
A derivation of the patch antenna is what is commonly known as a planar inverted F antenna, or PIFA. The PIFA can resonate at a much smaller patch size for fixed operating frequency as compared to the conventional patch antenna. It is generally a λ/4 resonant structure and is implemented by short-circuiting the radiating element to the ground plane using a conductive wall, plate or post. Thus, the conventional PIFA structure consists of a conductive radiator element disposed parallel to a ground plane and insulated from the ground plane by a dielectric material, usually air. This radiator element is connected to two pins, typically disposed toward one end of the element, giving the appearance of an inverted letter “F” from the side view. One pin electrically connects the radiator to the ground plane, the other pin provides the antenna feed. Impedance matching is obtained by selecting correct positioning of the feed and ground contacts. Thus, the conventional PIFA structure is similar to a shorted rectangular microstrip patch antenna.
These and other conventional antenna solutions offer good performance at attractive prices in relatively small packages. Despite these qualities, however, antenna designers continue to strive to improve operating efficiency, enhance multi-band operation, minimize losses resulting from capacitive tuning, and decrease the antenna's sensitivity to its surroundings.